context= dict is stored by reference and leaks between containers #568

This was generated by AI during a pre-4.0 audit of main at 9cd88e7.

Observation

container.py:89: ContextRegistry(context=context or {}) keeps the caller's dict. Reproduced:

ctx = {Req: r}
c1 = root.build_child_container(context=ctx)
c2 = root.build_child_container(context=ctx)
c1.set_context(User, "alice")
# c2 sees User; ctx now contains User

A module-level dict reused per request leaks set_context values across requests.

Proposal

Copy the dict on construction (dict(context)). Costs one small dict copy per child build; measure G6/G7. Observable change for code relying on the mutation, so before 4.0.

Acceptance

Failing test first; set_context on one container is invisible to siblings and to the caller's dict.